Isoprene may be obtained as a by-product from naphtha cracking by extraction from pyrolysis fractions or it may be synthesised by a number of routes. Synthetic routes to isoprene have been comprehensively reviewed in the literature (see, for example, M J Rhoad, Rubber Ind (London) 9 (12), 68 (1975); E Schoenberg, H A Marsh, S J Walters and W M Saltman, Rubber Chem Tech 52, 526 (1979) and D L Schultz, Ed, Rubber World 179 (7), 87 (1980)). Extraction, dehydrogenation of isoamylene or isopentane and synthesis from acetylene/acetone or isobutene and formaldehyde are the usual processes to obtain isoprene, commercially. A high proportion of commercial production is obtained by extraction.
Recently, interest has turned to a process for the synthesis of isoprene form linear butenes by isomerisation of mixed linear butenes, hydroformylation of the butene-2-mixture to 2-methylbutanal and dehydration of the 2-methylbutanal (see, for example, Proceedings 26th Assembly of International Rubber Study Group, Kuala Lumpur 29th Sep.-4 Oct. 1980, 218; D A Bol'shakov et al, Prom Sint Kauchuka 8, 2 (1980), European Patent No. 80449 and Forster, D; Sluka, J P and Vavere A Chemtech 16 No. 12 746, 1986). The commercial attraction of this process is the availability of large quantities of linear butenes as a raffinate int he production of methyl tertiary butyl ether (MTBE) as an octane booster for petrol. A process for producing isoprene by dehydration of valeraldehyde (2-methyl butanal) using aluminium silicate at 400.degree. to 600.degree. C. was first disclosed in U.S. Pat. No. 1,033,180 (1911). Much more recently, British Patent No. 1,385,348 (Erdolchemie) discloses a process for the catalytic production of dienes from aldehydes by dehydration using an acidic dehydration catalyst. Boron phosphate is a particularly preferred dehydration catalyst for the process. USSR Inventors Certificate No. 721116 (Bolshakov et al) discloses a method for the preparation of boron phosphate for the production of isoprene, the invention relating to a method of obtaining a boron phosphate catalyst with enhanced activity and selectivity. Other improved boron phosphate catalysts and/or improved processes for their use in the dehydration reaction have been described in U.S. Pat. Nos. 4,524,233, 4,587,372, 4,632,913 and 4,628,140 (all Goodyear Tire & Rubber) and U.S. Pat. No. 4,547,614 (Monsanto). The conversion of 2-methylbutanal to isoprene using boron phosphate catalysts has been extensively studied by Moffat and co-workers (see, for example, J B Moffat Rev. Chemical Intermediates 8 1 (1987)). Although boron phosphate catalysts exhibit high conversions and selectivities, they deactivate very rapidly and long term effective reactivation techniques have not been disclosed in the literature. Catalysts, other than boron phosphate, which have been investigated for this dehydration reaction include an alum or a mixed sulphate derived therefrom (British Patent No. 2,063,297) and magnesium ammonium phosphate (British Patent No. 2,093,060). Irodov, A V, Smirnov, V A and Kryrukov S I Zh Org Khim, 18 1401 (1982) describe the study of a number of catalysts based on the phosphates of aluminium, boron and calcium although the results reported relate only to calcium phosphate catalyst. A wide variety of catalysts including Be3(PO4)2, Mg3(PO4)2, Si3(PO4), Ba3(PO4) 2 and AlPO4 et al, was studied by Maraeva V A et al, Neftekhim 30 (2) 211 (1990). The use of zerolites as dehydration catalyst has been disclosed in U.S. Pat. No. 4,560,822 and European Patent No. 219, 042 (BASF) as well as U.S. Pat. No. 4,734,538 and European Pat. No. 272,662 (Union Carbide).